As shown in FIG. 1, a conventional PWM converter 10 operative to provide an output voltage Vout switchable between two levels includes a power stage 12 driven by a PWM signal to produce an inductor current IL to charge an output capacitor Cout at an output node 16, a control circuit 14 to generate the PWM signal according to a feedback signal VFB at a feedback node 18, a resistor R1 coupled between the output node 16 and a feedback node 18, a resistor R2 coupled between the feedback node 18 and a ground node GND, and a resistor R3 and a switch MS serially coupled between the feedback node 18 and the ground node GND. The feedback signal VFB contains the information of the converter output voltage/current when the PWM converter 10 is enabled. The feedback signal VFB is also one of the parameters which affect the duty-cycle, switching frequency, on-time and off-time of the PWM converter 10. A rapid change of the feedback signal VFB will cause a rapid change of the converter output voltage/current.
FIG. 2 is waveform diagram of the PWM converter 10 shown in FIG. 1 during an up transition of the PWM converter 10, in which waveform 20 represents the feedback signal VFB, waveform 22 represents the output voltage Vout, and waveform 24 represents the inductor current IL. To switch the output voltage Vout from a lower level to a higher level, as shown at time t1, the switch MS is turned on so that the resistor R3 is parallel coupled to the resistor R2. As a result, the feedback signal VFB drops abruptly and instantly, as shown by the waveform 20. When the feedback signal VFB is lower than a reference value, the PWM converter 10 must charge the output capacitor Cout immediately in order to achieve the best output response. Therefore, the PWM converter 10 will charge the output capacitor Cout by its maximum slew-rate, thereby increasing the output voltage Vout as shown by the waveform 22. When the feedback signal VFB catches up the reference value, as shown at time t2, the inductor L gets more energy than steady state. This energy will mainly be transferred to the output node 16, and thereby causes output overshoot.
FIG. 3 is waveform diagram of the PWM converter 10 shown in FIG. 1 during a down transition of the PWM converter 10, in which waveform 30 represents the feedback signal VFB, waveform 32 represents the output voltage Vout, and waveform 34 represents the inductor current IL. The switch MS is switched from on state to off state to switch the output voltage Vout from a higher level to a lower level. In response thereto, the feedback signal VFB jumps abruptly and instantly, as shown at time t3. Consequently, the feedback signal VFB becomes higher than a reference value, and the PWM converter 10 has to discharge the output capacitor Cout. When the feedback signal VFB down close to the reference value, as shown at time t4, the inductor current IL is usually less than the steady state current, and the difference between this inductor current IL and the steady state current will cause undershoot on the output voltage Vout. As shown by FIGS. 2 and 3, an abrupt, rapid change on the feedback signal VFB will make the PWM converter 10 over-react.
Conventionally, the approaches to relieve the overshoot/undershoot of a PWM converter focus on the application circuits. The most commonly used approaches are (a) to lower the inductance, and therefore when doing the transition, there will be less energy stored in the inductor L; (b) to enlarge the output capacitor and thereby get a slow slew rate on the output voltage Vout; and (c) to add low pass filters on the feedback node 18 to prevent rapid feedback signal changes. However, they all rely on adjustments outside the controller chip.
Therefore, it is desired an on-chip undershoot/overshoot eliminator for a PWM converter.